Lost against a 1850 and a 1750 in the exact same silly fashion by overlooking a pin followed by a double attack on the pinned piece. I was playing White, the pawn next to my king had been moved, opening the diagonal for Black’s black-square bishop, and each time I stepped right into a classic pin winning the exchange. But because I’m an idiot, I ignored the threat, trying to counter-attack, and on the next move lost the whole rook because I allowed my opponent to attack the pinned piece again. Basic tactics stuff. Blunders come in packs, I suppose. The basic chunk (Qe3? … c3??):

And for good measure, the most common chunks for bishop and castled king: